1. Field of the Invention
This invention relates to a demodulator for demodulating frequency-modulated (referred to as FM hereinafter) signals, and more particularly to a demodulator for demodulating FM video signals reproduced from a magnetic recording medium in a magnetic recording and reproducing apparatus such as a video tape recorder.
2. Description of the Prior Art
In the conventional video tape recorder (referred to as VTR hereinafter) and the like, an FM signal demodulating circuit demodulates the reproduced FM video signal by the so-called pulse counter method to obtain the original video signal. The reproduced FM video signal is passed in turn through a limiter to form a rectangular pulse-train signal like a compressional wave as shown in FIG. 1(a), through a differentiator to form a pulse-train signal as shown in FIG. 1(b), where it is further full-wave rectified to form a signal as shown in FIG. 1(c), and finally through an integrator to form a demodulated signal which is substantially equivalent to the original video signal as shown in FIG. 1(d). This signal processing is explained in detail in "Video Recording Techniques" by Nobutoshi Kihara, published by Sanpo Shuppan, Japan.
Since the carrier frequency of the FM video signal is very close to the higher side frequency of the video signal of VTR, a part of the FM video signal, when demodulated, becomes a crosstalk signal appearing in the original video signal, resulting in fine streaks or beats appearing on the display screen. For avoiding this crosstalk, in the conventional FM signal demodulating circuit, full-wave rectification is employed to double the carrier frequency, so that the crosstalk components of the FM video signal in the original video signal are suppressed. Usual home VTRs operate under a condition in which their video frequency band is 1.5 MHz, and their frequency deviation is 1.0 MHz, and their carrier frequency is 3.9 MHz. Therefore, the first and second lower sidebands do not cause crosstalk. However, if the conventional FM signal demodulating circuit is used in a VTR requiring a high resolution and a high S/N, or having a wide video transmission band and a large frequency deviation or emphasis, the second and third lower sidebands appear in the video signal as crosstalk signals, so as to cause deterioration of the picture quality. Moreover, in the conventional FM demodulating circuit in which the carrier frequency is doubled, the frequency deviation is doubled, so that powers of the second and third lower sidebands increase, resulting in production of fine streaks and beats, notably, a deterioration of the S/N at the edge of video signal.